1. Field of the Invention
The present invention relates generally to the fabrication of graphite sputtering targets and more specifically to the segmentation, orientation, and bonding of pyrolytic graphite sputter targets.
2. Description of Related Art
Sputter targets are used on a cathode sputtering apparatus to apply films or coatings of the target material on a substrate. Cathodic sputtering refers to the erosion of the cathode target material by ion bombardment that occurs when an electrical discharge is passed between electrodes in a low pressure gas. In the sputtering process inert gas ions with a positive charge are accelerated from the glow discharge, that forms between the electrodes, to the negative cathode. Erosion results from the ejection of atoms and clusters of atoms from the target surface as a result of momentum transfer from the bombarding ions. Sputtering becomes a coating process when the ejected material is deliberately condensed on a substrate suitably positioned near the cathode.
Magnetron sputtering is an effective method for producing carbon coatings from graphite targets. Magnetron sputtering uses a magnetic field to trap electrons in a region near the target surface creating a higher probability of ionizing a gas atom. The high density of ions created near the target surface causes material to be removed many times faster than in diode sputtering. The magnetron effect is created by an array of permanent magnets included within the cathode assembly that produce a magnetic field normal to the electric field.
Ion bombardment not only causes atoms of the target material to be ejected, but it also imparts considerable thermal energy to the target. Consequently, the target material must provide for adequate thermal transfer of the target's heat to the cooling media. This is particularly true in the case of magnetron sputtering where very large ion currents are produced causing a very intense and localized heating of the target.
Prior-art carbon sputter targets are typically made from plates of high-purity hot-pressed graphite. Although this material is high in chemical purity, it generally possesses about 15% porosity, which also introduces defects into the carbon films it produces. A more attractive material for carbon sputter targets is high-purity fully-dense pyrolytic graphite. However, the anisotropic heat transfer properties of pyrolytic graphite have prevented its use in the past as a carbon sputter target material. In the present invention a segmented sputter target design, with directionally oriented pyrolytic graphite pieces that are bonded together, engineers around the heat transfer problem to allow the use of pyrolytic graphite as a target material.
Carbon films deposited by magnetron sputtering are used in a variety of commercial and scientific applications. The quality of the carbon film can be adversely affected by the growth of carbon whiskers on the carbon target during the sputtering process. An observed correlation exists between the number of defects in the carbon film and the amount of whisker growth in the erosion area of the sputter target. The effect of sputtering process parameters on whisker growth has been studied and was published by Chen, et al. in "Surface-Defect Formation in Graphite Targets During Magnetron Sputtering (J. Vac. Sci. Technol. A8(4), Jul./Aug. 1990). Whisker growth in a carbon target results in defects in the sputtered carbon film produced. Selection of an appropriate target design and the type of carbon in the sputter target can significantly reduce the amount of whisker growth during magnetron sputtering.
Since sputtering is a momentum transfer process where atoms of a target are ejected by striking the target surface with ions, most of the energy in the sputtering process converts to heat in the sputter target. This heat is removed by directly or indirectly cooling the sputter target. The anisotropic thermal conductivity of pyrolytic graphite makes efficient cooling by such means difficult. The limitations in the prior-art synthesis of pyrolytic graphite targets produces a target material with the low thermal conductivity c-axis normal to the target's surface. Because the high thermal conductivity a-axis and b-axis of the target material are parallel to the target's surface, heat is conducted along the target faster than it is conducted away from the target. The result is target overheating that causes inefficient sputtering of the pyrolytic graphite.
U.S. Pat. No. 4,136,213 teaches the vapor deposition of electrically conductive carbon onto oxide supports to produce electrodes. U.S. Pat. No. 4,551,216 describes a sputtering process for producing carbon films on a substrate where the coating layer is thin and possesses low conductivity. U.S. Pat. No. 4,767,517 shows a sputtering process for producing diamond-like thin film coatings. Japanese Patent 24515 teaches the fabrication of dielectric targets for sputtering comprised of a number small blocks assembled together and bonded to an electrode. There is no mention of producing or using pyrolytic graphite layers in any of these inventions.